The present invention relates to a thin film semiconductor device and a semiconductor substrate sheet to be used in the semiconductor device as well as a method for producing them.
As is well known, a thin film semiconductor device or thin film transistor (TFT) comprises a substitute, in which a thin film layer of semiconductor materials such as silicon is formed on a base layer of insulation materials such as non-alkaline glass, or quarts glass. In the thin film layer of semiconductor, a plurality of channel consisting of a source area and a drain area are formed and each of channels is equipped with a gate electrode. Generally, the thin film layer of semiconductor consists of amorphous or polycrystalline silicon. However, a TFT using a substrate comprising a thin film layer of amorphous silicon cannot be used for a device which requires a high speed operation owing to its extreme low mobility(usually, approximately less than 1 cm2/V sec). Therefore, recently, a substrate comprising a thin film layer of polycrystalline silicon is used in order to increase the mobility. Nevertheless, even in a case of using such substrate, the improvement of mobility is limited because of such phenomenon at the time of operation as dispersion of electron at boundaries between crystal grains, owing to the fact that the polycrystalline thin film consists of numerous crystalline grains of extreme small size.
Thus, it has been tried to obtain a substrate having a thin film layer which makes it possible to increase mobility by avoiding disadvantageousness such as electron dispersion, by means of making the size of polycrystalline silicon to be large. For instance, it has been tried to obtain a thin film layer having a semiconductor grains of about 1 xcexcm size and having a mobility of about 100 cm2/V sec., by annealing a layer of polycrystalline silicon in a high temperature furnace. However, the above process has a disadvantage that inexpensive glass sheets such as sodium glass sheets cannot be used and expensive quartz glass sheets which can bear high temperature should be used, as the process requires an annealing by extreme high temperature such as over 1000xc2x0 C. A substrate using such expensive materials is not suited for producing a device of wide size screens in view of costs.
Some other trials has been proposed in order to obtain a thin layer which consists of polycrystalline semiconductor of large size grains, by means of irradiating a thin film of amorphous or polycrystalline semiconductor with energy beams such as excimer laser, instead of using high temperature annealing. By this method, it is possible to enlarge the size of a crystal grain, by using inexpensive glass sheets as the base layer.
Nevertheless, even by the method using irradiation of excimer laser, the size of obtained crystal grain could not exceed 1 xcexcm and it is inevitable that sizes of grains become uneven. For instance, in the specification of JPA 2001-127301, there is described a technology for obtaining a thin film layer of polycrystalline semiconductor of large size crystal grains comprising following steps; that is, polycrystalline a thin film of amorphous silicon by a melt recrystallization method, for example, using excimer laser irradiation, then depositing a thin layer of amorphous silicon on the recrystallized layer and, crystallizing the whole layers by solid phase growing method, thereby growing original polycrystalline grains to large size grains. However, in the above technique, it is suggested that the maximum size of obtained crystal grain is about 1000 nm(1 xcexcm) and sizes are uneven (cf. FIGS. 2 to 5 in the above specification)
Furthermore, there is an important problem, which has been neglected in the technique of semiconductor device comprising polycrystalline semiconductor, that is, the problem of arrangement mode of crystal grains. In the thin layer of polycrystalline semiconductor produced by previous techniques, the arrangement mode of crystal grains in the two-dimensional direction is utterly random. No trial for making such arrangement of crystal grains to be a regulated mode has been made. But, randomness of arrangement of crystal grains causes a serious disadvantage.
That is, it would be needless to say that the numerous units of transistor circuits formed in a thin film semiconductor device have to be arranged in a regulated mode such as geometrical arrangement mode. Therefore, when the sizes of crystal grains are uneven and the arrangement thereof are random (not regulated) in a thin film layer, an unit circuit of one transistor is inevitably set in such a modem to extend to a plurality of crystal grains of various sizes and positions (cf. FIG. 6). This would bring such result that the mobility and the electron transfer mode of each unit circuit are different one another, and this in turn would bring a bad influence to the quality of the device. As the result, when characteristics of every unit circuits differs each other, a device cannot but be designed on the whole by being based on the low level characteristics. This is an important problem to be solved.
It is an object of the present invention to provide a substrate sheet for thin film semiconductor devices, in which a plurality of large size single crystalline grains of semiconductor are formed in a regulated arrangement mode such as a matrix-arrayed configuration, thereby making it possible to use it as the substrate sheet for a thin film semiconductor device in which an unit circuit comprising a source electrode, a drain electrode and a gate electrode is formed on each of crystal grain.
It is further object of the present invention to provide a thin film semiconductor device, which has a high mobility without being influenced by disadvantage such as unevenness of crystal grain size or electron dispersion occurred in crystal grain boundaries, by means of setting an unit circuit comprising a source electrode, a drain electrode and a gate electrode on each of crystal grains which are arranged regulatedly in such mode as a matrix-arrayed configuration in the thin film layer of semiconductor.
It is another object of the present invention to provide a process for producing a substrate sheet for a thin film semiconductor devices in which a plurality of large size single crystalline grains of semiconductor are formed in a regulated arrangement mode such as a matrix-arrayed configuration.
It is another further object of the present invention to provide a process for producing a thin film semiconductor device in which an unit circuit comprising a source electrode, a drain electrode and a gate electrode are formed on each of crystal grains which are arranged regulatedly in such mode as a matrix-arrayed configuration in the thin film layer of semiconductor.
Thus, the substrate sheet for thin film semiconductor device of the present invention comprises; a base layer of insulation materials, a thin film layer of semiconductor formed on the base layer, a plurality of single-crystalline semiconductor grains formed in the thin film layer of semiconductor and, said plurality of single-crystalline semiconductor grains being arranged in a regulated configuration such as a matrix-arrayed configuration in the thin film layer of semiconductor.
The thin film semiconductor device of the present invention comprises; a base layer of insulation materials, a thin film layer of semiconductor formed on the base layer, a plurality of single-crystalline semiconductor grains formed in the thin film layer of semiconductor, said plurality of single-crystalline semiconductor grains being arranged in a regulated configuration such as a matrix-arrayed configuration and, each of said single-crystalline semiconductor grains being equipped with an electric circuit comprising a gate electrode, a source electrode and a drain electrode.
The method for producing a substrate sheet for thin film semiconductor devices according to the present invention comprises following steps; namely,
(a) forming a thin film semiconductor layer of non-single-crystalline semiconductor such as amorphous or polycrystalline semiconductor on a base layer of insulation materials and,
(b) crystallizing or recrystallizing said non-single-crystalline semiconductor to produce a plurality of single-crystalline semiconductor grains by irradiating it with energy beams, said irradiation being carried out so that irradiated points to which maximum irradiation intensity is given and irradiated points to which minimum irradiation intensity is given are arranged in a regulated configuration such as a matrix-arrayed configuration.
The process for producing a thin film semiconductor device of the present invention comprises following steps; namely,
(a) forming a thin semiconductor layer of amorphous or polycrystalline semiconductor on a base layer of insulation materials,
(b) crystallizing or recrystallizing said amorphous or polycrystalline semiconductor to produce a plurality of single-crystalline semiconductor grains by irradiating it with energy beams, said irradiation being carried out so that irradiated points to which maximum irradiation intensity is given and irradiated points to which minimum irradiation intensity is given are arranged in a regulated configuration such as a matrix-arrayed configuration,
(c) forming a gate electrode on each of single-crystalline grains in the thin film semiconductor layer, which has been produced by said step (b) and,
(d) fabricating an electric circuit in each of said single-crystalline semiconductor grains by forming a source electrode and a drain electrode therein.